Windtre for SIM card

Have you checked your own cell provider or their international plan? (example- Verizon is $10 day for cell service ONLY on days you use it, or $100 for a mont I believe)
We find that keeping our phone in Airplane Mode and just using free wifi everywhere is all we need. You can text and get emails, use the internet, etc.
Regardless- you do not have to use the app to purchase tickets- you can buy at the window or a kiosk. The airport has free wifi as well.
WhatsApp is another option.

More info if you search the forum
https://search.ricksteves.com/?button=&date_range=1y&filter=Travel+Forum&query=Sim+Italy+&utf8=%E2%9C%93

I’ve been to Italy many times and find that either VodaFone or TIM simcards work very well.
My phone doesn’t support eSIM either, and I like to be able to make voice calls.
Just buy your train tickets to Florence at the ticket machines or at an actual ticket desk at the airport.
Don’t buy a simcard at an airport.
If you have trouble you can go back to an actual store for help.
Then you don’t have to worry.
Buy your simcard in Florence.
There is a TIM store to the slight right of SMN train station when you come out facing the big area where you’ll see buses etc., on Piazza Della Stazione.
There is a Vodafone store opposite “Eataly” just near the Duomo and Baptistry, on Via de Martelli.
Just look for them on Google maps.
This year I used Vodafone.
About E30 for more data than you could ever hope to need.
They will take a copy of your passport.

We have boost mobile, and they did introduce an international plan recently but don't think it's worth it because there customer service sucks dirtand don't wanna take chances. Can't use apps on wifi without data, right? I thought we had to buy tickets & reserve our seats for the train in advance, thus needing to validate our tickets via app. If we wait until we land in Milan and buy at the window, in June of a holy year, isn't that a big gamble there may not be any seats left? That's really the only reason I'm even considering buying at the airport, to be able to use the train.

I think you need to do a bit more research on train travel in Italy as well as the need for data vs wifi.
As I said above we have found we can get by quite well just using free wifi w/phone on airplane mode.

If you feel you must have cell service/data for your whole trip then do as SJ has suggested and wait til you are in Florence to purchase your SIM. Be sure you understand how that works- you will have a new Italian phone number- not your US number.

Can't use apps on wifi without data, right?

Yes- you can use apps with just wifi. I suppose some options may not work as well but you do not need data to use apps.

I thought we had to buy tickets & reserve our seats for the train in
advance, thus needing to validate our tickets via app. If we wait
until we land in Milan and buy at the window, in June of a holy year,
isn't that a big gamble there may not be any seats left?

No- this is the opposite of what you should do. You do not want to purchase train tickets in advance for your arrival day- unless you are willing to eat the cost if you miss that train. Just buy the tickets when you get to Milan- no need for an app- buy at the window or at kiosk.
There are multiple fast trains from Milan to Florence- almost every hour- the chances of all those trains (or even any) selling out is highly unlikely.

That's really the only reason I'm even considering buying at the
airport, to be able to use the train.

This doesn’t make any sense. Of course you can use the trains if you don’t have the app or data service.

I see so many things online and even this forum saying to buy in advance asap especially for high speed trains.. see below for just one example of many you can easily search. Am I insane?

https://community.ricksteves.com/travel-forum/tours/when-to-buy-train-tickets-to-venice-from-florence

Way in advance is the cheapest price, but if you miss that train you lose your money and will have to buy another ticket at full price. We took this risk once, landing in Milan and taking the train straight to Venice and it worked out, but we were willing to forfeit the money if our plane was late. It's not about there being enough seats on the train.

No you are not insane you are just reading it wrong
As above buying in advance gives you discounts but you take a risk -there are flight delays, you can’t predict how long passport control and baggage will take, etc

Buying in advance for other train trips during your trip is fine as long as you are set on dates and times

Fast trains rarely sell out and there are many

Windtre is just the Italian outlet for the British cell company 3. Quite reputable.

Have a good read first of this website about Italian trains:

https://www.seat61.com/train-travel-in-italy.htm

It’s kept up to date and is invaluable to prepare yourself for train travel.
Only the very tiniest and remote train stations are unlikely to have staff and ticket kiosks.
The big ones: Milan, Rome, Florence, Bologna etc are all fully staffed and will help you with buying tickets correctly.
The staff seem to have become more friendly and helpful over the years I’ve been taking Italian trains, making it much easier for first time travelers.

Oh thank goodness, that makes more sense now. I was really afraid to wait thinking that was part of the reason people say book on advance and, after looking back it does appear to be older threads talking about availability. I am still a bit concerned with it being Holy year, but what to do. Grazie mille!